1. Field of the Invention
The invention relates to an X type electric welding machine comprising at least two gun arms each having electrodes at tip ends thereof which oppose each other wherein at least one gun arm is rotatably pivoted on a connection part (6) between the two gun arms, particularly to a rotating mechanism of the gun arms.
2. Description of Related Art
Conventionally, there has been an X type welding machine having first and second gun arms wherein the first gun arm is integrated with a gun bracket and the second gun arm is rotatably pivoted on the gun bracket so as to obtain a high electrode pressing force while restraining the capacity of the servomotor, and wherein the second gun arm presses against the first gun arm by a crank mechanism (pressing mechanism) composed of an output disc fixed to the output side of a reduction gear by way of a servomotor and the reduction gear, a pin provided at the outer periphery of the output disc, and a connector arm (link) provided so as to connect between the pin and the rear end of the second gun arm (see e.g., JP-A 9-248678).
However, in rotating means of the gun arms of the conventional X type electric welding machine, a pressing operation is performed at the position immediately before an angle formed between a line connecting between the center of the output disc by way of the reduction gear and the pin provided at the outer periphery of the output disc and a ring connecting between the rear end of the second gun arm and the pin becomes 180 degrees, thereby increasing the pressing force utilizing a so-called toggle mechanism.
Meanwhile, in such pressing means, the variation rate of the electrode pressing force relative to the variation of rotational angle of the output disc is large in view of the relation of the output of the link relative to rotational torque of the output disc, and further, the electrode pressing force is varied not linear functionally but exponential functionally as the angle set forth above approaches to 180 degrees so that the electrode pressing force is greatly varied by a slight variation of the angle formed between the pin and the link. In the practical use of a welding machine, the deflection of the gun arms is varied for every application force in addition to the variation of the wear of the electrode, and the wear of bearings for supporting pins including a main shaft so that it is difficult to obtain a stable angle. Accordingly, in view of a toggle mechanism where the pressing force is exponential functionally varied relative to the amount of variation of the rotational angle of the output disc, it is very difficult to control for obtaining an accurate electrode pressing force conforming to a set value of an electrode pressing force in the conventional X type electric welding machine.